Microstrip balun



Aug. 4, 1970 B. F.-GUNSHI'NAN ET AL 3,523,260

MICRQSTRIP BALUN Filed Aug. 18, 1969 IN VENTORS:

BERNARD E GUNSHINAN BERNARD LEE BUIFTUN ATTORNEY "United States Patent 3,523,260 MICROSTRIP BALUN Bernard F. Gunshinan, Sepulveda, and Bernard L.

Burton, Simi, Calif., assignors to The Bendix Corporation, a corporation of Delaware Filed Aug. 18, 1969, Ser. No. 850,888 Int. Cl. H03h 7/38; 1101p 3/08 US. Cl. 333-46 Claims ABSTRACT OF THE DISCLOSURE A miniature hybrid microstrip balun is described for providing an efficient coupling means between conductors such as aSO-ohm coaxial conductor and a ISO-ohm microwave antenna. Using microstrip techniques, a hot side conductor track tapered according to a Tchebycheff pattern is associated with a wide, but tapered, ground plane conductor on opposite sides of a dielectric substrate. Since the impedance transformation effected by this means provides only a portion of that required at the point where the hot side conductor is tapered to a minimum practical width, this conductor is maintained at this minimum width and the ground plane conductor is tapered smoothly down to a comparable width, thus providing the remaining impedance transformation and at the same time accomplishing mode transformation without significant reflective losses or phase shift.

BACKGROUND OF THE INVENTION There are many applications for which broad band impedance matching devices are needed. In addition to impedance matching, such devices must effect mode transformations, as the signal is transferred, for instance, from a coaxial lead to microstrip to an antenna with a minimum of reflective losses. Space limitations, especially on aircraft, often require that antennas and antenna feed devices be miniaturized to the greatest extent possible. The device which has been developed to accomplish a smooth transition between balanced two-conductor lines and an antenna is known as a balun transformer or, simply, a balun. A balun has been defined as an impedance matching transition from coaxial line to a balanced twoconductor open line. This definition and some theory and background of balun transformers is set forth in an article by J. W. Duncan and V. P. Minerva entitled. 100:1 Bandwidth Balun Transformer published in Proceedings of the IRE, February 1960. This article describes a balun transformer made by cutting tapered opening in the outer wall of a coaxial cable so that a series of crosssectional views show progressively large sectors removed from the outer conductor. The structures described in this article did not meet applicants particular requirement for a balun operative in the 2-12 gHz. frequency range, largely because of extreme space limitations, and because following the theory directly would lead, in this frequency range, the impractically small conductors.

SUMMARY OF THE INVENTION The device described herein is a hybrid microstrip balun using two basically different techniques to accomplish an impedance transformation over the desired impedance range. Two printed circuit conductors separated by a substrate having a substantial dielectric constant are connected at one end to a coaxial conductor and at the opposite end to a pair of wires which feed an antenna. Over a portion of the impedance range the transformation is accomplished by means of microstrip techniques in which the hot side (center lead) of the coaxial line is connected to a printed circuit conductor having a smooth tapered characteristic such as a T chebycheff taper "ice which gives a smooth response and essentially no phase shift over the pass band, and the outer conductor is connected to a ground plane of sufficient area as to give operation similar to a theoretical infinite ground plane. Other tapered conductor configurations may be used so long as reflective and radiation losses are minimized. When the conductor on the hot" side was tapered down to a minimum practical width (about .005"), there remained a substantial impedance ditferential between the balun and the antenna. A dilferent technique was required to effect this transition. It was found that the desired additional impedance change could be effected by narrowing the ground plane conductor until it was at essentially the same width as the hot" side conductor. A discontinuity in the ground plane conductor at the junction between the microstrip section and the remaining secton was still found to produce excessive losses through reflections, etc. It was then found that if the ground plane conductor were given a smooth taper over the entire length of the balun, such losses were substantially reduced. Thus the balun produced is a hybrid device wherein printed circuit patterns are applied to opposite sides of a medium dielectric substrate which, for approximately one-half of its length, uses a microstrip arrangement with the Tchebychelf taper, and the remaining half involves a two-lead impedance matching technique using a straight tapered ground line. Even for operation in the 2-12 gHz. frequency range, the balun, although small, is mechanically simple to manufacture, requiring only reasonable tolerances and straightforward production techniques.

DESCRIPTION OF THE DRAWINGS FIG. 1 is an enlarged plan view of one side of a microwave balun employing our invention.

FIG. 2 is an enlarged plan view of the opposite side of the balun shown in FIG. 1.

FIG. 3 is an enlarged end view of the balun shown in FIGS. 1 and 2.

FIG. 4 is a perspective view of the device shown in FIGS. 1 through 3 in a typical installation including a coaxial connector and mounting panel.

FIG. 5 is a perspective view of the assembly of FIG. 4 as seen from the opposite side.

DESCRIPTION OF THE PREFERRED EMBODIMENT FIG. 1 shows a balun incorporating our invention which includes a substrate 10 of medium dielectric material such as alumina. A conductive track 12 is attached to the substrate 10. For the frequency range of interest to applicants, the alumina substrate 10 was made about one inch long and about .OZS-inch thick. The conductor is typically of gold which has the desirable qualities of good conductivity and relative freedom from oxidation. Other dielectric materials and conductors may be used, and those skilled in the art are aware of the considerations involved in matching the dielectric thickness and the conductor width and materials to the frequency range and environmental conditions to be expected.

Approximately the lower half of the conductive track 12, as shown in FIG. 1, is shaped and proportioned to provide the Tchebycheff response as described above. The amount of taper available was limited, however, because if the conductor were made narrower than shown, its width would have been reduced below .005" which would be impractically small for mechanical reasons. Thus, the impedance transformtaion over this range is from about 50 ohms to about ohms which, in this instance, was only about half that desired. The upper half (approximately) of the conductor track, as shown, thus is a straight section which cooperates with the conductor on the opposite side to produce the desired impedance and mode change.

FIG. 2 shows the reverse side of the balun with the substrate 10 carrying a tapered conductor 14 constituting a microstrip ground plane. The upper half (approximately) of conductor 14 cooperates with the lower half of conductor 12 to produce the impedance transformation described above. Because of the mode change from a microstrip technique to a two-wire configuration in the center of the unit, is was found that any bend or discontinuity in the ground plane conductor 14 produced undesirable reflective losses; so the tapered conductor 14 was arrived at which maintains at its upper end sufficient area to act as a ground plane for the cooperating tapered conductor and yet tapers smoothly to achieve the desired impedance transformation at the lower end. Some degree of curvature in the sides of the ground plane conductor will not adversely affect operation to any great extent, but abrupt discontinuities, such as bends or notches, will produce losses of some magnitude.

FIG. 3 is an end view of the balun shown in FIGS. 1 and 2 but with the thickness of the conductive tracks considerably exaggerated since such plated conductors are typically about 150 microns in thickness. The view is taken from the wide end of the balun.

Opposite sides of the balun thus far described are shown in connection with an installation to a bulkhead or panel in FIGS. 4 and 5. In FIG. 4 a coaxial connector 16 is fastened through a hole drilled in a panel or bulkhead 18. The lower side of the connector is extended to form a flat shelf 20 and the balun is supported on thisshelf such that the center conductor 22 of the coaxial connector 16 is in contact with conductor 12 and the extended shelf 20 is in contact with the ground plane conductor 14. This latter connection is shown more clearly in FIG. which depicts the opposite side of the balun. The conductors 12 and 14 are connected to a pair of lead wires 24 and 26, respectively, which provide connecting means to an antenna not shown.

Obviously, the teachings herein may be adapted to devices which will appear physically quite different. The size is closely related to the frequency range of interest, although the balun described above is efficient over a rather broad frequency range extending from about 2 gHz to above 12 gHz. The thickness of the dielectric substrate will vary with the frequency and the dielectric constant of the material used.

What is claimed is: 1

1. A miniature microstrip balun comprising a high dielectric substrate having two essentially parallel spaced sides, a first conductor fastened to a first side of said substrate, said conductor being tapered over approximately half its length with its greatest Width at the one end of said substrate and tapering to its minimum width near its center and with said minimum Width maintained over the remainder of the length of said substrate, and a second conductor fastened to the opposite side of said substrate, said second conductor having substantial width at said one end of said substrate and tapering smoothly toward the opposite end of said substrate to a width approximately the same as theminirnurn width of said first conductor.

2. A microstrip balun as set forth in claim 1 wherein said first conductor is tapered to provide Tchebychetf tive and radiation losses and essentially no phase shift.

3. A microstrip balun as set forth in claim 2 wherein said first conductor is tapered to provide Tchebycheif response characteristics.

4. A microstrip balun as set forth in claim 1 wherein said second conductor is of sufiicient area to act as a ground plane for said tapered portion of said first conductor.

5. A miniature microstrip balun transformer for matching a coaxial conductor having a center lead and a ground conductor to a microwave device comprising:

a high dielectric substrate having opposite essentially parallel sides and having one end wider than the other; I

a first conductor track fastened to one of said sides and extending the length of said substrate, said track including a first portion having its greatest width at thewidest end of said substrate and gradually tapered to a minimum width to provide a Tchebychelf response characteristic, and a second portion constituting a continuation of said conductor at said minimum width; and

a second conductor track fastened to the other of said sides having a width at the wide end of said substrate substantially greater than the widest part of said first conductor and tapering smoothly to a width approximately the same as said minimum width.

References Cited UNITED STATES PATENTS 3,419,813 12/1968 Kamnitsis 33334 ELI LIEBERMAN, Primary Examiner M. NUSSBAUM, Assistant Examiner US. Cl. X.R. 

